Method of making formulated plastic separators for soluble electrode cells

ABSTRACT

A method of making a membrane comprised of a hydrochloric acid-insoluble sheet of a mixture of a rubber and a powdered ion transport material is disclosed. The sheet can be present as a coating upon a flexible and porous substrate. These membranes can be used in oxidation-reduction electrical accumulator cells wherein the reduction of one member of a couple is accompained by the oxidation of the other member of the couple on the other side of the cell and this must be accompained by a change in chloride ion concentration in both sides. The method comprises preparing a mixture of fine rubber particles, a solvent for the rubber and a powdered ion transport material. The mixture is formed into a sheet and dried to produce a microporous sheet. The ion transport material includes particles ranging from about 0.01 to 10 microns in size and comprises from 20 to 50 volume percent of the microporous sheet.

ORIGIN OF THE INVENTION

This invention was made by an employee of the United States Governmentand may be manufactured or used by or for the Government without thepayment of any royalities thereon or therefor.

This is a continuation of application Ser. No. 776,146, filed Mar. 10,1977, now U.S. Pat. No. 4,133,941.

BACKGROUND OF THE INVENTION

The development of means of storing bulk quantities of electrical powerhas become increasingly important in recent years. Redox cells, havingsoluble electrodes in both of the charged and discharged states, havebeen the object of increased interest as a method for the efficientstorage of electrical energy. Such redox cells could store energygenerated from time dependent energy sources such as solar electric andwindmill electric installations. Examples of redox cells which can beused in connection with the present invention are described in detail inthe technical paper "Electrically Recharbable Redox Flow Cells" byLawrence H. Thaller, NASA TM X-71540. Additionally, such flow cells aredescribed in U.S. Pat. No. 3,996,064 which is hereby incorporated byreference. In their simpiest embodiments, these redox flow cells containtwo storage tanks, each containing one of the two metal ions, togetherwith chloride anions which make up the redox couple. Although almost anyredox couple can be used in such a cell, considerations regardingefficiency can be made in the choice of the particular couple. Thus,systems such as the Fe⁺² /Fe⁺³ //Ti⁺³ /TiO⁺² and the Fe⁺² /Fe⁺³ //Cr⁺³/Cr⁺² systems are preferred. Thus, the cathodic fluid, e.g., aqueousconcentrated Fe⁺³, and the anodic fluid, e.g., aqueous concentratedTi⁺³, are fed from their respective tanks through the redox flow cellwhere the system can either be charged from an external source or can bedischarged to release the stored electrical energy. The power that canbe withdrawn from or put back into the system depends on many factorsincluding the tank volumes, the flow rates and the electrochemicalfeatures of the particular redox couple utilized and the characteristicsof the electrode compartments.

In a two tank system employing multiple passes of the fluid, the fluidwould constantly be recycled after passage through the fuel cell. In afour tank system, the fluids would pass from their respective tanksthrough the fuel cell and then into two other storage tanks. The systemcould then be electrically recharged by applying a suitable voltage tothe terminals of the power conversion section as the fluids are pumpedback up to the original tanks.

A two-tank system is shown in the drawing. The anodic fluid and cathodicfluid tanks are represented by 1 and 2, respectively. Pipelines 3 forthe feeding operation to the redox flow cell are provided from thetanks. The redox flow cell 4 contains inert electrodes 5 and a selectivemembrane 6. After passing through the cell, the fluids are returned totanks 1 and 2 by pumps 7.

The costs of the electrical accumulator installations have been thesubject of much interest as indicated by the technical paper "Cost andSize Estimates for an Electrochemical Bulk Energy Storage Concept" byMarvin Warshay and Lyle O. Wright, NASA TM X-71805. Deployment of theelectrical accumulator system utilizing a redox cell system with a solarpower source has been the subject of the technical paper "The Redox FlowSystem for Solar Photovoltaic Energy Storage" by Patricia O'Donnel,Randall F. Gahn and William Pfeiffer, NASA TM X-73562.

U.S. Ser. No. 707,124, filed July 20, 1976, now U.S. Pat. No. 4,018,971,of which the inventor of the present application is co-inventor,describes the use of hydrochloric acid-silica gels as an ion transportmedium for use in the membrane separating the couple compartments ofsuch redox flow cells.

The membrane utilized must provide an impermeable barrier to the cationsof the particular couple utilized. However, the membranes must bepermeable to the extent of maintaining the charge neutrality of eachcompartment by the migration of the anion used through the membrane. Itshould be noted that fuel cells can be designed with either anions orcations migrating through the membrane to provide the neutralizationrequired. However, there is an inherent disadvantage from an energystandpoint in moving cations from the anode compartment during dischargeas opposed to moving anions from the cathode compartment. That is, if ahydrogen cation is required to migrate during discharge, one mole ofhydrochloric acid is required per Faraday over and above any acid thatmay be required for pH adjustment needed for solution stabilization.

Therefor, redox fuel cells that use anion migration through the membraneare somewhat preferred. Anions that can be used with the gels of thepresent invention include halide ions such as chloride and bromide.

Materials that have been used as anion-permeable membranes includepolymers such as IONAC 3475. Such membranes can be made by grinding aquaternary ammonium ion-exchange resin to a powder and then polymerizinga monomer in the present of the thus-formed powder. Additionally, a webcan be used in conjunction with the membranes to provide support.

Among the prior art membranes are those described in U.S. Pat. No.3,497,389 issued to Carl Berger et al. These membranes utilize inorganicadditives of controlled water vapor characteristics capable of retainingwater and providing water vapor pressures above 100° C. The additivescan be mixed with the ion conducting material, e.g., zirconiumphosphate, granulated and pressed into discs which are then sintered.

Further, prior art structures include ceramic membranes such as thosedescribed in U.S. Pat. No. 3,392,103 issued to Carl Berger.

Other separators for use in different types of batteries have beendescribed in U.S. Pat. No. 2,816,154 to Mendelsohn and 3,018,316 toHiggins.

An object of the present invention is a membrane that can be used inbulk electrical energy storage systems which has a low ionicresistivity, high selectivity for anions as opposed to metal cations andlow electronic conducitivity.

A further object of the invention is a membrane for use in a redox flowcell which exhibits minimal increases in resistivity with the passage oftime and which prevents the accumulation of unequal amounts of waterbetween the two sides of the redox cells.

SUMMARY OF THE INVENTION

It has now been found that a rubber-ion transport material sheetingformulated with or without a flexible and porous substrate possessesdesirable characteristics as the membrane between the acid compartmentsof a redox couple for use in a bulk electrical energy storage system.

The present invention is a method of making a microporous sheet for useas such a membrane. According to the method a mixture of fine rubberparticles, a solvent for the rubber and a powdered ion transportmaterial of particle size from about 0.01 to 10 microns is prepared. Themixture is formed into a sheet and then dried to provide a microporoussheet. The ion transport material comprises from about 20 to 50 volumepercent of the microporous sheet.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE shows an electrically rechargeable bulk power storage systemwith the anion transport membrane 6 of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The rubber based resin or polymer used in the sheeting of the presentinvention can be any of the well known thermoplastic rubbers which areresistant to concentrated hydrochloric acid. This property isnecessitated by the presence of concentrated hydrochloric acid in bothof the redox flow cell couple compartments. Since concentrations ofhydrochloric acid ranging from 0.5 to 6.0 normal are generally used inthe redox flow cells, the rubber utilized can be satisfactorilyqualitatively tested by immersing a block of the rubber in 6.0 normalhydrochloric acid overnight and observing any deterioration. Thus, therubber can be a block copolymer selected from the group of thermoplasticrubbers consisting of chains of three blocks, e.g., an elastomeric blockin the center and a thermoplastic (polystyrene) block on each end. Theelastomeric mid block is an ethylene-butylene rubber. Other types ofblock copolymers have polybutadiene or polyisoprene midblocks.Additionally, rubbers such as a copolymer of styrene andethylene-butylene rubber can be used. One such copolymer is Kraton GX7050. The rubber component of the sheeting is usually compounded withthe other ingredients after being cut into fine particles in order tofacilitate solution. Alternatively, the rubber can be cut into asolution with the solvent only utilizing a high speed blender beforethis mixture is added to the other ingredients.

The sheeting comprising the rubber and the ion transport material can beused per se or can be used in the form of a laminate wherein it ispresent as a coating upon a flexible and porous substrate as will beexplained hereinafter. In any case, the sheeting is prepared by mixingthe rubber, ion transport material and solvent, pouring this mixture inthe form of a layer and thereafter drying the layer by allowing thesolvent to evaporate.

The solvent that is used need only be one that can completely dissolvethe rubber and allow the ion transport material to also become dissolvedor suspended. Although this dissolving of the rubber appears to resultin a solution, this may not be true in a strict chemical sense.Chlorinated hydrocarbon solvents have been found to be useful in thepresent invention with trichloroethylene or chloroform being preferred.However, essentially all solvents which can dissolve rubber to theextent necessary to completely intermix the rubber and the ion transportmaterial can be used in the present invention. Thus, toluene has beenfound to be useful as the solvent, a rubber such as Kraton GX 7050 goinginto toluene slightly easier than into trichloroethylene.

The substrate with which the sheeting can be used must be flexible andporous but can be any desired thickness. Fuel-cell grade asbestos sheetabout 10 mils thick has been found to be very satisfactory and asuitable fuel-cell grade sheet can be obtained from the Quinn-T Company.Ordinary news print, when coated with the sheeting mixture of thepresent invention makes an excellent separator for use between the redoxcell compartments. Further, additional substrates can be a variety ofmicroporous sheets about 10 to 20 mils in thickness, e.g., the A-05 orA-10 foamed polyvinyl chloride-filled sheets obtained from AmeraceCorporation of Butler, N.J. A further substrate is the Kimberly-ClarkKimcloth having a weight of 1 to 2 ounces per square yard which is anembossed poly-propylene non-woven mat having a thickness of about 6 to12 mils. An additional substrate can be a Dacron cloth. It is apparentfrom the above listing that a substantial variety of flexible and poroussubstrates can be used in the present invention.

As defined herein, a chloride ion transport membrane is one which whenused with a metal couple wherein oxidation and reduction takes place,respectively on either side of the membrane, the fuel cell will chargeor discharge properly and will not be impeded by the inability of themembrane to equalize the charges on either side thereof. Thus, in aTiCl₃ -TiCl₄ and FeCl₃ -FeCl₂ system, the former is the anolyte whilethe latter is the catholyte. On discharge, FeCl₃ is reduced to FeCl₂while TiCl₃ is oxidized to TiCl₄. The ion exchange membrane allows thepassage of chlorine ions, i.e., chloride, from one compartment to theother to preserve electroneutrality. However, this can also take placeby the passage of H⁺, these two situations being summarized below:##STR1##

The ion transport material is used in an amount of from about 50 to 20volume % as compared with about 50 to 80 volume % of the rubber.Preferably, the amount of the ion transport material is about 30 to 35volume %. Utilization of more than about 50 volume % of the iontransport material may result in the creation of holes greater than 100angstroms which can cause intermixing of the two couple solutionsresulting in a lower efficiency of the redox cell.

The ion transport material should be insoluble in 6 Normal hydrochloricacid, the term "insoluble" being used herein to denote the situationwhere no solubility can be observed and the situation where thesolubility is extremely low.

Preferred among the ion transport materials are salts of a chlorideanion and a phosphonium, tertiary ammonium or quarternary ammoniumcation, a metal oxide, a silicate or boric acid. Especially preferredamong these materials are the hydrochlorides of cross-linkedpoly-4-vinyl pyridine and polybenzimidazole.

The following specific materials have been used in the membranes of thepresent invention:

1. Amberlite IRA-400;

2. aniline hydrochloride;

3. crosslinked poly 4-vinyl pyridine;

4. ethylene tris (2-cyanoethyl) phosphonium bromide;

5. polymerized triazine resins;

6. titanium dioxide;

7. magnesium zirconium silicate;

8. polybenzimidazole;

9. boric acid;

10. colloidal montmorillonite;

11. synthetic saponite;

12. synthetic hectorite;

13. Amberlite IRA-120.

Although the above ion transport materials are used as fine powders insizes ranging from about 0.1 to 10 microns in diameter, the titaniumdioxide mentioned above is utilized as a powder in sizes ranging from 15to 40 millimicrons in diameter. After the membranes are prepared asdried sheetings, they are immersed in 6 Normal hydrochloric acid for atleast 4 hours in order to convert any salts to the chlorides salts andin order to convert any neutral amines to the chloride salts. However,other normalities can be used.

Tertiary amines have been found to have very stable electricalresistance in the membranes of the present invention while quaternaryamines tend to have increasing resistance with time but have a desirablyhigh output initially.

Although it is resonably certain that the ammonium chlorides salts actas ion transport materials by the transport of chloride ions in themembranes of the present invention, materials such as titanium dioxidehaving areas of 50 to 100 square meters per gram may be exhibiting anadsorption phenomon and may actually be transporting H⁺ cations.Therefore, the chloride anion transport membranes of the presentinvention are defined as being suitable separators for redox flow cellshaving chloride anions in hydrochloric acid solutions whereby the flowcell can be charged, discharged and thereafter continuously recycled forextended periods of time.

The membranes of the present invention may additionally contain a liquidtertiary amine, secondary amine or mixture thereof in an amount up toabout 10% by weight of the ionic transport material. This addition tendsto improve the electrical output of the membrane. However, these aminesshould be of sufficient molecular weight to be substantially insolublein the acid solution.

In order to prepare a membrane of the present invention, the rubber,solvent and ion transport material are placed in a bulk mill and milledover night. However, it may be found desirable, especially with theKraton rubber mentioned above, to cut the rubber into solution with ahigh speed blender before being put into the bulk mill. When the millingis complete, the material viscosity is adjusted, if necessary, by theaddition of solvent in order to give a viscosity range of about 15 to 20seconds with a No. 3 Zahn cup at 25° C.

The thus-produced mixture can then be formed into a dried sheet, appliedas a coating to a flexible and porous substrate or formed into a driedsheet and thereafter laminated to a flexible and porous substrate byconventional means. If applied to a substrate to form a dried coating,it can be applied to one or both sides. Further, it can be applied toone side and two of the membranes thus-formed can be utilized togetheras the membrane of the cell.

Further, different coatings can be applied to two separate flexiblesheets and the combination of two membranes thus-formed can be used as amembrane of the fuel cell. It has surprisingly been found that thecharging and discharging characteristics of the cell will differaccording to the orientation of the thus-formed laminate with respect tothe two couple compartments. For example, it has been found that it ispreferable to have a tertiary amine coating on the iron side with aquaternary amine on the titanium side of a Fe-Ti metal couple asdescribed above. This finding is described more fully in Examples 6 and7.

Which ever method is used for the formation of the membrane, theevaporation of the solvent has suprisingly been found to give rise tomicropores in the surface less than about 100 A and this is believed tobe one of the reasons for the excellent characteristics of the membranesof the present invention.

After the milling is completed, the material is applied to either aflexible and porous substrate or to a surface from which the layer canbe later removed. Knife-coating procedures can be used for thisoperation and the coating should be about 10 to 40 mils in wetthickness, preferably about 25 mils. Alternatively, the coating may beapplied by using a roller or by dipping the substrate in the mixture.After drying overnite to accomodate solvent release, the coating is thenabout 1.5 to 20 mils thick and is flexible and microporous as explainedabove. A second coating can then be applied if desired. The finishedmembrane separator is then cut to a size for testing in a cell andsoaked in a hydrochloric acid solution of normality comparable to thenormality of the redox cell into which the membrane will be inserted.The soaking should take place for at least 4 hours in order to replacethe anion therein with chloride anion and to convert neutral speciesinto chloride salts. Then the membrane is placed into the cellconsisting of a polycarbonate case, graphite electrodes, graphite clothand rubber gaskets for sealing. The cell is then filled with 6 normalhydrochloric acid and the resistance of the assembly is measured with abridge at 1000 Hertz. The resistances or impedances usually range fromabout 0.20 to 0.80 ohms. The cell is then drained and is placed in theelectrical accumulator system with storage tanks and pumps for thecouple solutions and a known resistance of 1 ohm through which thedischarging system can release energy.

In order to test the membrane and as tested in the following examples,the couple tanks are filled with about 40 ml of 1 Molar FeCl₃ in 0.5 HClon one side and about 40 ml of 1 Molar TiCl₃ in 6 Normal HCl on theother side (40 ml of 1 Molar solution being approximately the equivalentof 1 ampere-hour). A 1 ohm resistor is placed in the electrical circuitalong with an ampere-hour intergrator and a power supply for charging.The pump is turned on and the solutions are allowed to circulate througheach side of the cell for at least 2 hours to permit mixing of thesolution in each respective system and wetting of the electrode andmembrane circuit with solution. When the open circuit voltage andresistance is stabilized, the power characteristics of the membrane aredetermined by measuring the voltage-to-current ratio during applicationof an ever increasing electronic load. From this E/I plot, a maximumpower is determined and the resistance where it occurs is calculated.These values are used for comparing one membrane to another.

Cycle testing can be conducted upon the above-described fuel cell. Thetest consists of an initial discharge of solutions followed by chargingand discharging cycles for a period of time from several weeks up toseveral months. During this time, the rate of solution cross-mixingacross the membrane is determined by chemical analysis and theresistance is constantly measured to monitor stability. The voltage andcurrent as well as ampere-hours are measured and recorded each cycle tocheck for losses and comparison of these losses to solution mixingrates. Also during this cycle testing, the membrane should be tested inorder to determine whether it causes a self-discharging of the chargedsolutions.

During several months of testing, the membrane resistance shouldincrease only slightly, e.g., about 0.1 to 0.2 ohms. More desirably, theresistance should remain level or should perhaps even decrease slightly.Additionally, during this cycle testing, the time to one halfconcentration of each solution is calculated. Any valve above 1,000hours indicated an especially desirable membrane.

Further during this cycle testing, the change of solution volume in thereservoirs is observed and recorded. It is undesirable to have movementof water across the membrane and the membranes of the present inventionshould have a difference in volume after several months of less than20%.

In this cycle, the charging is referred to as a "tapered current charge"while the discharge is referred to as a "resistive discharge".

The following non-limiting examples illustrate the preparation of themembranes of the present invention.

EXAMPLE 1

40 grams of finely-diced Kraton GX 7050 rubber was added to a high speedblender containing about 700 grams of trichloroethylene and mixing wasconducted until a homogeneous solution was obtained. The solution wasthen added to a ball mill and milled over night with 20 grams ofcross-linked poly 4-vinyl pyridine, obtained from Reilly Tar andChemical Co. of Indianapolis, Ind., and 22 grams of magnesium zirconiumsilicate powder, obtained from NL Industries of Hightstown, N.J. Theamount of solvent was adjusted to give a viscosity range of 15 to 20seconds with a No. 3 Zahn cup at 25° C. The volume percent ofingredients excluding the solvent is 65 for GX 7050; 28 for cross-linkedpoly 4-vinyl pyridine; and 7 for MgZrSiO₃.

The material was then applied to an asbestos sheet obtained from theQuinn-T Company in an amount of about 25 mils wet thickness. The wetcoating was then allowed to dry overnight at room temperature.

The thus-produced membrane was then cut into a 2 inch by 2 inch squareand immersed in a 6 Normal hydrochloric acid solution for 4 hours.

This membrane gave acceptable performance by the criticia mentionedabove for a period of about 500 hours of cycle testing operation.

EXAMPLE 2

A membrane was prepared as indicated in Example 1 with magnesiumzirconium silicate with the substitution of 21 grams of AmberliteIRA-400 in place of the pyridine compound. The IRA-400 is a quaternaryamine of chloromethylated styrene-divinylbenzene copolymer and wasobtained from Rohm and Haas Company.

The IRA-400 strong base anion exchanger is obtained in bead form and ispowdered before use. This is accomplished by mixing equal weights of thebead and water in a high speed blender for about 5 minutes followed byallowing the water to evaporate. The powder has a size of about 5 to 10microns. The volume percent of ingredients excluding the solvent is thesame as for Example 1.

This membrane was cycled tested as indicated above for 2500 hours withlittle change in membrane performance.

EXAMPLE 3

A membrane was prepared as in Example 1 with the substitution of 28grams of titanium dioxide (DeGussa P-25) for the ion transport materialsof Example 1. The volume percent of ingredients excluding the solvent is60 for GX 7050 and 40 for TiO₂.

When cycled tested, this membrane gave acceptable performance for 500hours.

EXAMPLE 4

A membrane was prepared as set forth in Example 1 with the substitutionof 24 grams of the quaternary amine of Example 2 for the ion exchangematerials of Example 1. The volume percent of ingredients excluding thesolvent is 67 for GX 7050 and 33 for IRA-400.

When cycled tested as indicated above, this membrane gave acceptableperformance for 500 hours.

EXAMPLE 5

A membrane was prepared according to Example 1 with the exceptions that17 grams of the crosslinked pyridine was used with no magnesiumzirconium silicate. The volume percent of ingredients excluding thesolvent is 72.5 for GX 7050 and 27.5 for cross-linked poly 4-vinylpyridine.

When cycled tested as indicated above, this membrane gave acceptableperformance for 500 hours.

EXAMPLE 6

A membrane was prepared wherein different formulations were applied toopposite sides of an asbestos sheet.

On a first side of an asbestos sheet identical to that used in Example1, a formulation containing cross-linked poly 4-vinyl pyridine andmagnesium zirconium silicate, prepared identically to the procedure inExample 1, was applied and dried. On the second side a formulationcontaining Amberlite IRA-400 and magnesium zirconium silicate, preparedidentically to the procedure in Example 2, was applied and dried.

The thus-prepared membrane was cycle tested with the test apparatusindicated above after being immersed in a 6 N HCl solution for 4 hours.

With the first side facing the Fe ion solution, results surprisinglysuperior to the opposite configuration were obtained. With an opencircuit, the resistance was stable at only 0.8 ohms, indicating lowresistance of the membrane itself. The T_(1/2), the time during cycletesting at which 1/2 of the Fe ions have migrated to the Ti side of themembrane, was found to be an acceptable 4700 hours. In addition, theresistance did not increase during this cycle testing.

With a second membrane thus-produced, and with the second side facingthe Fe ion solution, the T_(1/2) was found to be 3500 hours but initialopen circuit resistance was 1.5 ohms and this increased at the rate ofabout 0.03 ohms per day.

EXAMPLE 7

A membrane was prepared as in Example 6 with different formulations onopposite sides of an asbestos sheet.

A first formulation was prepared with Amberlite IRA-400. The bead formIRA-400 was mixed with an equal weight of water and mixed in a highspeed blender for about 5 minutes. After the water was allowed toevaporate, 24 grams of powder having a size of about 5 to 10 microns wasadded to a homogeneous solution of 40 grams Kraton GX 7050 prepared asin Example 1. The solution was then ball-milled overnight and the amountof trichloroethylene was adjusted as in Example 1.

The second formulation was preparing by adding 17 grams of thecross-linked poly 4-vinyl pyridine used in Example 1 to the Kraton GX7050 homogeneous solution of Example 1, ball-milling overnight andsolvent adjustment thereafter as in Example 1.

After application of the first and second formulations to the asbestossheet followed by drying and immersing in HCl a above in each case, twoof the thus-produced membranes were cycle tested, with the differentorientations to the Fe and Ti solutions.

With the first formulation facing the Ti solution side, the T_(1/2) was5600 hours but the stable resistance was 1.15 ohms and increased in thistest at the rate of 0.02 ohms per day.

With the second formulation facing the Ti solution side, the T_(1/2) was2100 hours. The stable resistance was about the same as above butincreased at the rate of 0.044 ohms per day.

It will understood that various modifications and adaptions of theinvention can be made by those skilled in the art without departing fromthe spirit of the invention and, accordingly, the invention is not takenas limited except by the scope of the following claims.

What is claimed is:
 1. A method of making a chloride anion transportmembrane comprising preparing a mixture of a rubber, a solvent for saidrubber and a powdered ion transport material, providing a layer of themixture and removing the solvent to produce a dried sheet, said sheetcomprising about 50 to 80 volume percent rubber and about 50 to 20volume percent of said ion transport material, said ion transportmaterial having particle sizes ranging from about 0.01 to 10 microns,said sheet having micropores less than 100 angstroms in diameter, saidion transport material being a salt of a chloride anion and aphosphonium, tertiary ammonium or quaternary ammonium cation.
 2. Amethod of making a chloride anion transport membrane comprisingpreparing a mixture of a rubber, a solvent for said rubber and apowdered ion transport material, providing a layer of the mixture andremoving the solvent to produce a dried sheet, said sheet comprisingabout 50 to 80 volume percent rubber and about 50 to 20 volume percentof said ion transport material, said ion transport material havingparticle sizes ranging from about 0.01 to 10 microns, said sheet havingmicropores less than 100 angstroms in diameter, said ion transportmaterial being a metal oxide, a silicate or boric acid.
 3. The method ofclaim 2, wherein said metal oxide is titanium dioxide and said silicateis montmorillonite, saponite, hectorite or magnesium zirconium silicate.4. A method of making a chloride anion transport membrane comprisingpreparing a mixture of a rubber, a solvent for said rubber and apowdered ion transport material, providing a layer of the mixture andremoving the solvent to produce a dried sheet, said sheet comprisingabout 50 to 80 volume percent rubber and about 50 to 20 volume percentof said ion transport material, said ion transport material havingparticle sizes ranging from about 0.01 to 10 microns, said sheet havingmicropores less than 100 angstroms in diameter, said ion transportmaterial being a mixture of magnesium zirconium silicate with eithercross-linked poly 4-vinyl pyridine or a quaternary amine ofchloromethylated styrene-divinylbenzene copolymer.